1. Field of the Invention
The present invention relates to an improved process for hardening microcapsules containing hydrophobic oil droplets, the microcapsules comprising a coacervate wall film of hydrophilic colloids. More particularly, in the production of microcapsules by a process of complex coacervation using at least gelatin as one hydrophilic colloid and a dialdehyde as a hardening agent, it relates to a process for hardening to thereby prevent a rapid increase in viscosity and yellowing of a capsule solution upon reaction between gelatin and glyoxal or glutaraldehyde after a hardening pretreatment step, particularly in a hardening treatment step.
As used herein, the term "hardening pretreatment" is intended to cover the operation of rapidly converting the pH of the coacervation system to the alkaline side to promote the reaction between gelatin and an aldehyde. The term "hardening treatment" is intended to cover the operation of raising the temperature of the system to further accelerate the reaction between gelatin and an aldehyde, thus forming a wall film of good heat resistance.
2. Description of the Prior Art
As a process for microencapsulating a hydrophobic oily liquid utilizing complex coacervation wherein one of hydrophilic colloids is gelatin, there is known the process which comprises the five steps of: (1) emulsifying a water-immiscible oil in an aqueous solution of at least one hydrophilic colloid ionizable in water (the first sol), and then admixing an aqueous solution of a hydrophilic colloid (the second sol) having an electric charge opposite to that of the first sol, or emulsifying a water-immisicible oil in an aqueous solution of hydrophillic colloids which are ionizable in water and at least one of which is positively charged [the emulsifying step]; (2) either adding water thereto or adjusting the pH thereof to cause coacervation, thus obtaining coacervates wherein a complex colloid is adhered to the individual oil droplets [the coacervation step]; (3) cooling the coacervates to cause gelation thereof [the gelation step]; (4) adjusting the pH to the alkaline side in the presence of a hardening agent, adding a hardening agent after adjusting the pH to the alkaline side, or adding both a hardening agent and an alkali at the same time [the hardening pretreatment step]; and (5) optionally elevating temperature of the system to harden the gelled coacervate [the hardening step].
However, microcapsules obtained by the above-described process are so unstable that they have the defect in the production thereof that hardening must be effected gradually by allowing them to stand for a long period of time under moderate conditions at a temperature lower than ordinary temperatures. This defect, however, has been somewhat improved by conducting a hardening treatment step subsequent to the aforesaid step (4), wherein oil-containing capsules are rapidly and completely hardened by gradually raising the temperature up to 40.degree.-60.degree.C.
As the agent for hardening the gelatin wall film, in general aldehydes such as formaldehyde, glyoxal, glutaraldehyde, etc., are used. Of these aldehydes, dialdehydes such as glyoxal, glutaraldehyde and the like rapidly harden the wall film and the resulting wall film possesses an excellent heat resistance. However, they have the defect that upon leaving them for a comparatively long period of time at a low temperature after the hardening pretreatment step, the viscosity of the system gradually increases, the capsules produced gradually aggregate, and the capsule solution itself yellows. If the system is not stirred, these defects become more serious. Thus, there are important problems to be solved in that the stability of a microcapsule slurry with the passage of time must be improved.
Also, the conventional process has the defect that upon gradually raising the temperature of the system so as to rapidly and completely harden the oil-containing capsules, the viscosity rapidly rises, the capsules aggregate to form a giant flock, and the capsule solution yellows deeply.
In addition, microcapsules produced by the conventional process have the defect that when used in a pressure sensitive paper, the resolving power of the colored letters in copying using a plurality of sheets is deteriorated because of the large particles. When coating on an original paper by means of an air knife coater, the air pressure of the coater must be raised due to the high viscosity of the coating solution. Thus, an increase in coating speed is hindered.